Display device

ABSTRACT

According to one embodiment, a display device includes a pixel electrode and a memory in each of pixels, a common electrode, a first drive circuit which supplies a digital signal, a second drive circuit which supplies an AC common signal to the common electrode, a storage control circuit which stores the digital signal in the memory in a storage period, and a select control circuit which selectively supplies, in a display period, to the pixel electrode, one of a display signal and a non-display signal. A frequency of the common signal in the storage period is a first frequency. The frequency of the common signal in the display period is a second frequency. The first frequency is higher than the second frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2015-212092, filed Oct. 28, 2015, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a display device.

BACKGROUND

Display devices, such as liquid crystal display devices in which amemory is provided in each pixel in a display area, are known. In thistype of display device, a storage period and a display period arealternately repeated. In a storage period, digital data based on theimage to be displayed is written to each memory. In a display period, animage is displayed in the display area by setting the drive potential ofeach pixel to potential corresponding to the digital data stored in acorresponding memory. The system for driving the pixels based on thedigital data stored in the memories in the above manner is called, forexample, a digital mode or a digital drive system.

In addition to the function of a digital mode, display devices havingthe function of an analog mode (or an analog drive system) to change thedrive potential of each pixel to a multilevel gradation are suggested.

To realize the operations in a storage period and display period,various types of circuits and switching elements are provided in eachpixel. When the switching elements are on and off, components such aspixel electrodes are electrically changed to a floating state in astorage period. The potential of the components in a floating statevaries according to the change in potential of other components. Becauseof this variation, an undesired electric field is generated in pixels.In this way, the brightness of images may be changed.

Display devices in which the above digital mode is employed are requiredto improve the display quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a structure common to each embodiment of adisplay device.

FIG. 2 is a plan view schematically showing a first substrate and asecond substrate provided in the display device.

FIG. 3 shows an example of the equivalent circuit of each subpixelprovided in the display device.

FIG. 4 is a timing chart showing an example of an operation performed inan analog mode by the display device.

FIG. 5 is a timing chart showing an example of an operation performed ina storage period by the display device.

FIG. 6 is a timing chart showing an example of an operation performed ina display period by the display device.

FIG. 7 is a timing chart shown for explaining an example of problemswhich can be caused in a digital mode.

FIG. 8 is a cross-sectional view showing each electric field produced ina subpixel in display and storage periods of the timing chart of FIG. 7.

FIG. 9 is a cross-sectional view showing each electric field produced ina subpixel in other display and storage periods of the timing chart ofFIG. 7.

FIG. 10 is a timing chart showing an example of an operation performedby a display device according to a first embodiment.

FIG. 11 is a timing chart showing an example of an operation performedby a display device according to a second embodiment.

FIG. 12 is a timing chart showing an example of an operation performedby a display device according to a third embodiment.

FIG. 13 is a timing chart shown for explaining other problems which canbe caused in a digital mode.

FIG. 14 is a timing chart showing an example of an operation performedby a display device according to a fourth embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a display device comprises: apixel electrode and a memory provided in each of a plurality of pixels;a common electrode which faces the pixel electrode; a signal line towhich a digital signal is supplied; a first drive circuit configured tosupply the digital signal to the signal line; a second drive circuitconfigured to supply an AC common signal to the common electrode; afirst drive line to which a display signal is supplied; a second driveline to which a non-display signal is supplied; a storage controlcircuit configured to store the digital signal supplied to the signalline in the memory in a storage period; and a select control circuitconfigured to selectively supply, in a display period, to the pixelelectrode, one of the display signal and the non-display signal, the onecorresponding to the digital signal stored in the memory. In the displaydevice, a frequency of the common signal in the storage period is afirst frequency. The frequency of the common signal in the displayperiod is a second frequency. The first frequency is higher than thesecond frequency.

Embodiments will be described hereinafter with reference to theaccompanying drawings.

The disclosure is merely an example, and proper changes in keeping withthe spirit of the invention, which are easily conceivable by a person ofordinary skill in the art, come within the scope of the invention as amatter of course. In addition, in some cases, in order to make thedescription clearer, the drawings are illustrated schematically, ratherthan as an accurate representation of what is implemented. However, suchschematic illustration is merely exemplary, and in no way restricts theinterpretation of the invention. In the drawings, reference numbers ofcontinuously arranged elements equivalent or similar to each other areomitted in some cases. In the specification and drawings, structuralelements which function in the same or a similar manner to thosedescribed in connection with preceding drawings are denoted by likereference numbers, detailed description thereof being omitted unlessnecessary.

In each embodiment, as an example of a display device, a reflectiveliquid crystal display device which has the function of an analog modefor driving pixels by display signals of multilevel gradation and thefunction of a digital mode as described above is disclosed. However, theembodiments do not prevent application of each technical idea disclosedin the embodiments to other types of display devices. Other types ofdisplay devices are assumed to be self-luminance display devices such asorganic electroluminescent display devices, or electronic paper displaydevices having cataphoresis elements, etc.

First, the structures and operations common to the display devices ofthe embodiments are explained with reference to FIG. 1 to FIG. 5.

FIG. 1 is a plan view showing an example of the general structure of adisplay device 1. The display device 1 comprises a first substrate SUB1,a second substrate SUB2 and a liquid crystal layer LC. The firstsubstrate SUB1 and the second substrate SUB2 are attached to each othersuch that they face each other. The liquid crystal layer LC is sealedbetween the first substrate SUB1 and the second substrate SUB2.

The display device 1 comprises a display area DA. The display area DA isequivalent to an area in which pixels PX are arranged in matrix on thefirst substrate SUB1. Specifically, a large number of pixels PX arearranged in matrix in a first direction X and a second direction Y inthe display area DA. The first direction X is, for example,perpendicular to the second direction Y. Each pixel PX includes red (R),green (G) and blue (B) subpixels SPX. In the present disclosure, thesubpixels SPX may be simply called pixels.

In the example of FIG. 1, the subpixels SPX included in one pixel PX arearranged in the first direction X. However, the layout of the pixels PXis not limited to the example of FIG. 1. For example, each pixel PX mayfurther include the subpixels SPX of other colors such as white (W). Atleast a part of the subpixels SPX included in one pixel PX may bearranged in the second direction Y.

The display device 1 further comprises a plurality of scanning lines G,a plurality of signal lines S, a control device 2, a scanning line drivecircuit 3 and a signal line drive circuit 4 (a first drive circuit). Thescanning lines G and the signal lines S are formed on the firstsubstrate SUB1. The scanning line drive circuit 3 and the signal linedrive circuit 4 are, for example, at least partially formed on the firstsubstrate SUB1, and are connected to the control device 2. The scanninglines G extend from the scanning line drive circuit 3 to the displayarea DA in the first direction X and are arranged in the seconddirection Y. The signal lines S extend from the signal line drivecircuit 4 to the display area DA in the second direction Y and arearranged in the first direction X. In a planar view, each signal line Spasses between subpixels SPX adjacent to each other in the firstdirection X.

For example, the control device 2 is an integrated circuit mounted onthe first substrate SUB1, and functions as a signal supply source whichoutputs various signals necessary for image display based on the imagedata input from outside. The control device 2 may not be mounted on thefirst substrate SUB1 or the second substrate SUB2. The control device 2may be connected to these substrates via a flexible wiring board. Thescanning line drive circuit 3 supplies a scanning signal to eachscanning line G in series. The signal line drive circuit 4 comprises amultiplexer 40. For example, the multiplexer 40 is a group of switchingelements for switching the output destination of signals between threesignal lines S connected to RGB subpixels SPX.

Each subpixel SPX comprises a memory 10 and a pixel electrode PE. Thememory 10 stores a digital signal supplied via a signal line S mainly ina digital mode. The pixel electrode PE faces a common electrode CEformed on the second substrate SUB2. The common electrode CE may beformed on the first substrate SUB1. The pixel electrode PE and thecommon electrode CE may be formed of a transparent conductive materialsuch as indium tin oxide (ITO). The common electrode CE is formed over aplurality of subpixels SPX. The common electrode CE is connected to anAC drive circuit 20 (a second drive circuit) provided in the controldevice 2 via a common electrode line LCM. An auxiliary capacitance lineLCS is also connected to the AC drive circuit 20. The auxiliarycapacitance line LCS extends to the display area DA and is connected tothe circuits of the subpixels SPX.

The display device 1 comprises a color filter facing each subpixel SPX.Each color filter has a color corresponding to the display color of thesubpixel SPX facing the color filter. The color filters are formed on,for example, the second substrate SUB2.

FIG. 2 is a plan view schematically showing the first substrate SUB1 andthe second substrate SUB2. A peripheral area FA is formed around thedisplay area DA. The peripheral area FA is equivalent to an areaexcluding the display area DA from an area in which the first and secondsubstrates SUB1 and SUB2 overlap each other in a planar view. Thedisplay device 1 comprises a light-shielding layer 5 overlappingsubstantially the entire peripheral area FA. The light-shielding layer 5is provided on, for example, the second substrate SUB2. By providing thelight-shielding layer 5, it is possible to prevent light leak from theperipheral area FA and reflection of light by the circuits or linesformed in the peripheral area FA.

A reflective layer 6 which reflects outside light is provided in thedisplay area DA. The reflective layer 6 may be formed of, for example, ametal material. The display device 1 displays an image, using the lightreflected by the reflective layer 6. The reflective layer 6 is incontact with one surface of the pixel electrode PE and is formed on thefirst substrate SUB1 as shown in, for example, FIG. 8 and FIG. 9explained later. The display device 1 may further comprise a front lightprovided on a surface of the second substrate SUB2 so as not to face thefirst substrate SUB1. The display device 1 may further comprise abacklight provided on a surface of the first substrate SUB1 so as not toface the second substrate SUB2. For example, a surface light sourcedevice which comprises a lightguide plate facing the display area DA anda plurality of light-emitting diodes provided along the end portion ofthe lightguide plate may be employed as the front light and thebacklight. Further, the display device 1 may comprise a backlightwithout comprising the reflective layer 6.

For example, the light-shielding layer 5 is not provided in the displayarea DA. The light-shielding layer 5 does not overlap each signal line Sprovided between adjacent subpixels SPX in the display area DA. In thisway, it is possible to increase the rate of opening of each pixel PX andrealize image display with a high brightness. The light-shielding layer5 may overlap a part of the display area DA. In this case, for example,the light-shielding layer 5 may be formed so as to overlap the scanninglines G.

FIG. 3 shows an example of the equivalent circuit of each subpixel SPX.Each subpixel SPX comprises the above pixel electrode PE, the abovememory 10, a gate circuit 11, a select control circuit 12 and a storagecontrol circuit 13.

The gate circuit 11 comprises switching elements Q1 and Q2. In switchingelements Q1 and Q2, the scanning line G is connected to the controlterminal, and the output terminal is connected to the pixel electrodePE. Switching elements Q1 and Q2 are, for example, double-gate thin-filmtransistors. A scanning signal GATEA is supplied to the scanning line Gsuch that switching elements Q1 and Q2 are on.

The select control circuit 12 comprises switching elements Q3 and Q4.The input terminal of switching element Q3 is connected to the signalline S. The input terminal of switching element Q4 is connected to theauxiliary capacitance line LCS. A display signal SIG or a first drivesignal xFRP is supplied from the signal line drive circuit 4 to thesignal line S. An auxiliary capacitance signal CS or a second drivesignal FRP is supplied from the AC drive circuit 20 to the auxiliarycapacitance line LCS. An auxiliary capacitance Csc for driving theliquid crystal layer LC is formed by the difference in potential betweenthe auxiliary capacitance line LCS and the pixel electrode PE. Further,the select control circuit 12 comprises a select signal line 12 aconnecting the output terminals of switching elements Q3 and Q4 to theinput terminal of switching element Q2. While switching elements Q1 andQ2 are on, the select signal line 12 a is electrically connected to thepixel electrode PE. While switching elements Q1 and Q2 are off, theselect signal line 12 a is electrically disconnected from the pixelelectrode PE.

In FIG. 3, the line extending from the AC drive circuit 20 branches tothe auxiliary capacitance line LCS and the common electrode line LCM. Inthis example, the auxiliary capacitance signal CS or the second drivesignal FRP supplied to the auxiliary capacitance line LCS has the samepotential as the common signal VCOM supplied to the common electrodeline LCM.

The memory 10 comprises switching elements Q5 to Q8. A first powersource line LP1 for supplying power source voltage VRAM is connected tothe input terminals of switching elements Q5 and Q7. A second powersource line LP2 to which voltage VSS is supplied is connected to theinput terminals of switching elements Q6 and Q8. For example, switchingelements Q5 and Q7 are PMOS transistors. Switching elements Q6 and Q8are NMOS transistors. The output terminals of switching elements Q5 andQ6 are connected to the control terminal of switching element Q4 suchthat a first CMOS inverter is structured. The output terminals ofswitching elements Q7 and Q8 are connected to the control terminal ofswitching element Q3 such that a second CMOS inverter is structured. Thefirst and second inverters are connected in parallel in oppositedirections and selectively set one of switching elements Q3 and Q4 to anon-state.

The storage control circuit 13 is a circuit for storing a digital signalin the memory 10, and comprises switching element Q9. The input terminalof switching element Q9 is connected to the signal line S. The outputterminal of switching element Q9 is connected to the control terminalsof switching elements Q5 and Q6. A digital scanning line LGD isconnected to the control terminal of switching element Q9. A scanningsignal GATED is supplied to the digital scanning line LGD.

All of switching elements Q1 to Q9 are, for example, thin-filmtransistors, and are formed on the first substrate SUB1. The auxiliarycapacitance line LCS, the scanning line G, the first power source lineLP1, the second power source line LP2 and the digital scanning line LGDare also formed on the first substrate SUB1, and are connected to aplurality of subpixels SPX arranged in the first direction X. Thesignals of the first power source line LP1, the second power source lineLP2 and the digital scanning line LGD are supplied from, for example,the control device 2.

The display device 1 having the above structure is capable of drivingeach subpixel SPX in both an analog mode and a digital mode. The analogmode is a mode for controlling the luminance of each subpixel SPX inmultilevel gradation based on the display signal supplied to the signalline S. The digital mode is a mode for controlling the luminance of eachsubpixel SPX in monochrome by simply controlling on and off based on thedigital data stored in the memory 10. In the explanation below, it isassumed that the display device 1 is a display device in anormally-black mode. It is assumed that, when the memory 10 is made high(high potential) in a digital mode, the subpixel SPX is on (whitedisplay). It is assumed that, when the memory 10 is made low (lowpotential), the subpixel SPX is off (black display).

The basic operation of the display device 1 in an analog mode and adigital mode is explained below.

(Analog Mode)

In an analog mode, a scanning pulse is supplied to each scanning line Gin series, and further, a display signal of multilevel gradation issupplied to each signal line S in series in accordance with the imagedata of a subpixel SPX corresponding to each scanning line G to which ascanning pulse is supplied. In this manner, potential based on imagedata is sequentially written to each group of subpixels SPX arranged inthe first direction X (hereinafter, referred to as each horizontalline).

FIG. 4 is a timing chart showing an example of an operation performed inan analog mode by the display device 1. The timing chart particularlylooks at the subpixel SPX shown in FIG. 3 and shows the change in thescanning signal GATEA supplied to the scanning line G, the displaysignal SIG supplied to the signal line S, the pixel potential PIX of thepixel electrode PE, the common signal VCOM supplied to the commonelectrode CE, the scanning signal GATED supplied to the digital scanningline LGD, the power source voltage VRAM supplied to the first powersource line LP1 and the memory potential RAM stored in the memory 10. Inthe explanation below, a period for writing the pixel potential PIX toone horizontal line is defined as a horizontal period TH.

In an analog mode, the memory 10 is made high. The operation for makingthe memory 10 high is the same as that of FIG. 5 as explained later.When the memory 10 is made high, and the power source voltage VRAM isincreased from voltage VDD to voltage VDD2 which is the drive voltage ofthe subpixel SPX, voltage VDD2 is supplied from the memory 10 toswitching element Q3. In this way, switching element Q3 is on. Switchingelement Q4 is off.

When the scanning signal GATEA of the scanning line G is increased fromvoltage VSS2 to voltage VDD2 (in other words, when a scanning pulse isinput), switching elements Q1 and Q2 are on. In this manner, the pixelelectrode PE is connected to the signal line S. At this time, as shownby the arrow in FIG. 4, the pixel potential PIX is set to the level ofthe display signal SIG of multilevel gradation supplied to the signalline S. After the scanning signal GATEA is decreased to voltage VSS2,the pixel electrode PE is in a floating state. The difference inpotential between the pixel electrode PE and the common electrode CE ismaintained by the auxiliary capacitance Csc. Thus, the subpixel SPXdisplays the color of gradation based on the written pixel potential PIXuntil the pixel potential PIX is rewritten next.

The example of FIG. 4 shows a case using line-inversion control forinverting the polarity of potential between the pixel electrode PE andthe common electrode CE for each horizontal line. Thus, the potential ofthe common signal VCOM is changed between voltage VSS and voltage VDDdepending on the horizontal period TH.

(Digital Mode)

In a digital mode, a storage period and a display period are repeated.In a storage period, a digital signal supplied to a signal line S isstored in the memory 10. In a display period, one of the first drivesignal xFRP and the second drive signal FRP is selected and supplied tothe pixel electrode PE so as to correspond to the digital signal (highor low) stored in the memory 10.

In a storage period, a scanning pulse is supplied to a digital scanningline LGD in series, and further, a digital display signal of ahorizontal line corresponding to the digital scanning line LGD to whicha scanning pulse is supplied is supplied to each signal line S inseries. In this way, a digital signal based on image data is written tothe memory 10 in series for each horizontal line.

FIG. 5 is a timing chart showing an example of an operation performed ina storage period by the display device 1. In a manner similar to that ofFIG. 4, the timing chart particularly looks at one subpixel SPX. In astorage period, the scanning signal GATEA of the scanning line G is setto voltage VSS2. Thus, the pixel electrode PE is set to a floatingstate.

In a horizontal period TH for writing data to the memory 10, the displaysignal SIG of the signal line S is set to the potential to be written tothe memory 10. It is assumed that a high voltage VDD corresponds towhite display, and a low voltage VSS corresponds to black display. Thepower source voltage VRAM of the first power source line LP1 isdecreased from voltage VDD2 to voltage VDD in a storage period so as tohave the same potential as the potential of the memory 10. When thescanning signal GATED of the digital scanning line LGD is increased fromvoltage VSS2 to voltage VDD2 (in other words, when a scanning pulse isinput), switching element Q9 is on, and thus, the memory 10 is connectedto the signal line S. At this time, as shown by the arrow in FIG. 5, thelevel of the display signal SIG supplied to the signal line S is writtento the memory 10. In the example of FIG. 5, high is written to thememory 10.

Subsequently, switching element Q9 is set to an off-state by decreasingthe scanning signal GATED to voltage VSS2. The power source voltage VRAMis increased to VDD2 which is the voltage for setting switching elementsQ3 and Q4 to an on-state. At this time, the voltage of the memory 10 isalso increased from VDD to VDD2. In this way, the memory 10 connects thefirst power source line LP1 and switching element Q3, and sets switchingelement Q3 to an on-state by the power source voltage VRAM. The memory10 connects the power source line LP2 and switching element Q4, and setsswitching element Q4 to an off-state by voltage VSS. Since switchingelement Q3 is on, the potential of the signal line S is supplied to thepixel electrode PE.

When the potential supplied to the memory 10 is low corresponding toblack display, the memory 10 connects the second power source line LP2and switching element Q3, and sets switching element Q3 to an off-stateby voltage VSS. The memory 10 connects the power source line LP1 andswitching element Q4, and sets switching element Q4 to an on-state bythe power source voltage VRAM. Since switching element Q4 is on, thepixel electrode PE is connected to the auxiliary capacitance line LCS.Thus, a signal having the same potential as that of the common signal issupplied. In the above manner, the memory 10 exclusively sets switchingelement Q3 or Q4 to an on-state by the stored voltage, and selects oneof the signal line S and the auxiliary capacitance line LCS as theconnection destination of the pixel electrode PE.

FIG. 6 is a timing chart showing an example of an operation performed ina display period by the display device 1. In a manner similar to that ofFIG. 5, the timing chart particularly looks at one subpixel SPX. Theexamples of FIG. 5 and FIG. 6 show cases using frame-inversion controlfor periodically inverting the polarity of potential between the pixelelectrode PE and the common electrode CE depending on the frame periodTF in all of the subpixels SPX arranged in the display area DA. Theoperation for rewriting the data of each memory 10 in each horizontalline constituting one frame is performed during, for example, one frameperiod TF. The series of horizontal periods TH shown in FIG. 5 areincluded in one frame period TF. In each frame period TF, the commonsignal VCOM and the auxiliary capacitance signal CS are constant. Bycontrast, as shown in FIG. 6, a display period includes a plurality offrame periods TF. The potential of the common signal VCOM and thepotential of the auxiliary capacitance signal CS change between voltageVSS and voltage VDD depending on the frame period TF.

In a display period, the auxiliary capacitance signal CS which changesdepending on each frame period TF is equivalent to the AC second drivesignal FRP. In a display period, the first drive signal xFRP is suppliedto the signal line S. The first drive signal xFRP is an AC signal havinga phase opposite to that of the second drive signal FRP, and changesbetween voltage VDD and voltage VSS depending on the frame period TF.

In a display period, the scanning signal GATEA of the scanning line G isincreased from voltage VSS2 to voltage VDD2. When switching element Q3is set to an on-state by the memory 10, the signal line S is connectedto the pixel electrode PE. When switching element Q4 is set to anon-state by the memory 10, the auxiliary capacitance line LCS isconnected to the pixel electrode PE. FIG. 6 shows an example in whichthe signal line S is connected to the pixel electrode PE, and thus, thepixel potential PIX is set to the first drive signal xFRP. In this case,a difference in potential as voltage VDD−voltage VSS is generatedbetween the pixel electrode PE and the common electrode CE. Thus, thesubpixel SPX is set to white display. When the auxiliary capacitanceline LCS is connected to the pixel electrode PE, no difference inpotential is generated between the pixel electrode PE and the commonelectrode CE. Thus, the subpixel SPX is set to black display.

As is clear from the above explanation, the signal line S has both afunction as a signal line supplied with the digital data to be stored inthe memory 10 and a function as the first drive line supplied with thefirst drive signal xFRP, which is the display signal of an image. Theauxiliary capacitance line LCS has both a function as a signal line forsupplying the auxiliary capacitance signal CS and a function as thesecond drive line supplied with the second drive signal FRP, which isthe non-display signal of an image. In this structure, the number oflines in the display area DA is reduced. Thus, it is possible to realizea high fineness for the subpixels SPX and improve the rate of opening.

In a storage period, switching elements Q1 and Q2 are off. Thus, thepixel electrode PE is set to a floating state. One of the problems to becaused by this situation is explained with reference to FIG. 7 to FIG.9.

FIG. 7 is a timing chart showing the change in the scanning signalGATEA, the common signal VCOM, the auxiliary capacitance signal CS(FRP), the display signal SIG (xFRP) and the pixel potential PIX in thestorage and display periods repeated through time. In this example, allof the subpixels SPX of the display area DA are continuously set towhite display. Here, one frame period TF constitutes a storage period.Two frame periods TF constitute a display period. However, more frameperiods TF may constitute a storage period and a display period.

In storage period 1 on the left side in the figure, the display signalSIG (solid line) of voltage VDD is supplied in order to make thepotential of the memory 10 high. In storage period 1, the scanningsignal GATEA is decreased to voltage VSS2. The pixel electrode PE is setto a floating state. Thus, the pixel potential PIX (dashed line) isdrawn to the increase in the potential of the common signal VCOM, andthus, is increased. The pixel potential PIX is increased to voltageVDD×2 such that the difference in potential between the pixel potentialPIX and the common signal VCOM in display period 0 immediately beforestorage period 1 is maintained. Voltage VDD×2 is the voltage which isapproximately twice the difference between voltage VDD and voltage VSS.

In display period 1 subsequent to storage period 1, the AC first drivesignal xFRP is supplied to the signal line S. In storage period 2subsequent to display period 1, the display signal SIG of voltage VDD issupplied again such that the memory 10 is kept high. In storage period2, the pixel electrode PE is in a floating state. Thus, the pixelpotential PIX is drawn to the decrease in the potential of the commonsignal VCOM, and thus, is decreased. Thus, the pixel potential PIX isdecreased to voltage −VDD such that the difference in potential betweenthe pixel potential PIX and the common signal VCOM in display period 1immediately before storage period 2 is maintained. Voltage −VDD is lessthan voltage VSS by approximately the difference between voltage VDD andvoltage VSS. Thus, in storage period 2, a large difference Vx inpotential is produced between the signal line S and the pixel electrodePE.

FIG. 8 is a cross-sectional view showing each electric field produced inthe subpixel SPX (a) in display period 0 immediately before storageperiod 1 and (b) in storage period 1. In this example, the reflectivelayer 6 is formed on a surface of the pixel electrode PE on the liquidcrystal layer LC side. The reflective layer 6 reflects, toward thesecond substrate SUB2, the light which has entered the second substrateSUB2, has passed through the liquid crystal layer LC and has reached thefirst substrate SUB1. In FIG. 8(a), the pixel electrode PE has voltageVDD, and the common electrode CE has voltage VSS. Thus, an electricfield from the pixel electrode PE to the common electrode CE isproduced. Since the signal line S has voltage VDD, an electric fieldfrom the signal line S to the common electrode CE is also produced.

In FIG. 8(b), the pixel electrode PE has voltage VDD×2. The commonelectrode CE and the signal line S have voltage VDD. Thus, an electricfield from the pixel electrode PE to the common electrode CE isproduced. In addition, an electric field from the pixel electrode PE tothe signal line S is produced.

For example, when voltage VSS is zero, and voltage VDD is 3.2 V, voltageVDD×2 is approximately 6.4 V. In this case, each electric field producedin FIG. 8(a) and FIG. 8(b) is caused by a potential difference of 3.2 V.

FIG. 9 is a cross-sectional view showing each electric field produced inthe subpixel SPX (a) in display period 1 immediately before storageperiod 2 and (b) in storage period 2. In FIG. 9(a), the pixel electrodePE and the signal line S have voltage VSS, and the common electrode CEhas voltage VDD. Thus, electric fields from the common electrode CE tothe pixel electrode PE and the signal line S are produced.

In FIG. 9(b), the pixel electrode PE has voltage −VDD. The commonelectrode CE has voltage VSS. The signal line S has voltage VDD. In thiscase, mainly, an electric field from the common electrode CE to thepixel electrode PE, and an electric field from the signal line S to thepixel electrode PE are produced.

For example, when voltage VSS is zero, and voltage VDD is 3.2 V, voltage−VDD is approximately −3.2 V. In this case, the electric field producedbetween the pixel electrode PE and the common electrode CE in FIG. 9(a)and FIG. 9(b) is caused by a potential difference of 3.2 V. The electricfield produced between the pixel electrode PE and the signal line S inFIG. 9(b) is caused by a potential difference Vx of 6.4 V and thus,strong. This strong electric field affects the vertical electric fieldbetween the pixel electrode PE and the common electrode CE as shown inthe area surrounded by the dashed circle in FIG. 9(b). Thus, theperformance of alignment control of the liquid crystal layer LC isreduced. In this way, the luminance of the subpixel SPX may be reduced.

On the bottom of FIG. 7, the display area DA (DISPLAY) which displays animage is schematically shown. It is assumed that all of the subpixelsSPX are continuously set to white display. Thus, the entire display areaDA is continuously set to white display in principle. However, instorage period 2, a flashing phenomenon in which the display area DA isdark in comparison with the other periods is generated because of thereduction in luminance caused by potential difference Vx. This flashingphenomenon leads to degradation in display quality.

In the above example, white display is explained. In a case of blackdisplay, similarly, a large difference Vx in potential is formed betweenthe pixel electrode PE and the signal line S in a storage period. Thus,a flashing phenomenon may be generated.

Hereinafter, this specification discloses various embodiments to preventthe degradation in display quality caused by a flashing phenomenon.

First Embodiment

In a first embodiment, when a display period transitions to a storageperiod, the potential of a common signal VCOM and a auxiliarycapacitance signal CS (FRP) in the display period is maintained in thestorage period. In this way, a flashing phenomenon is prevented. Thedetails of the present embodiment are explained below.

FIG. 10 is a timing chart showing an example of an operation performedby a display device 1 according to the first embodiment. FIG. 10 assumesthat each subpixel SPX is set to white display in a manner similar tothat of FIG. 7. FIG. 10 shows the potential of each signal in displayperiod 0, storage period 1, display period 1, storage period 2 anddisplay period 2 through time.

Immediately before storage period 1, in other words, in the second frameperiod TF of display period 0, the voltage of the common signal VCOM andthe auxiliary capacitance signal CS is VSS. In this case, an AC drivecircuit 20 maintains the voltage of the common signal VCOM and theauxiliary capacitance signal CS so as to be VSS in storage period 1. TheAC drive circuit 20 stops the AC output in storage period 1. In displayperiod 1 subsequent to storage period 1, the AC drive circuit 20restarts the AC output. In this way, the common signal VCOM and theauxiliary capacitance signal CS are AC signals (FRP) which changebetween voltage VSS and voltage VDD depending on the frame period TF.

Immediately before storage period 2, in other words, in the second frameperiod TF of display period 1, the voltage of the common signal VCOM andthe auxiliary capacitance signal CS is VDD. In this case, the AC drivecircuit 20 stops the AC output and maintains the voltage of the commonsignal VCOM and the auxiliary capacitance signal CS so as to be VDD instorage period 2. The AC drive circuit 20 restarts the AC output indisplay period 2 subsequent to storage period 2.

The waveform of a display signal SIG (xFRP) in FIG. 10 is the same asthat in FIG. 7. However, the waveform of pixel potential PIX in FIG. 10differs from that in FIG. 7. The pixel potential PIX is voltage VDD instorage period 1 since the common signal VCOM has voltage VSS. The pixelpotential PIX is voltage VSS in storage period 2 since the voltage ofthe common signal VCOM is VDD.

In the example of FIG. 7, the difference Vx in potential between a pixelelectrode PE and a signal line S is larger than the difference betweenvoltage VDD and voltage VSS in storage period 2. However, in the exampleof FIG. 10, the difference Vx in potential is within the differencebetween voltage VDD and voltage VSS. In this way, in the presentembodiment, a flashing phenomenon is prevented. Thus, the degradation indisplay quality can be prevented.

In this example, white display is explained. In a case of black display,similarly, a flashing phenomenon can be prevented. In the presentembodiment, the AC output of the common signal VCOM is stopped in bothstorage period 1 and storage period 2. However, the AC output may bestopped only in one or some of the storage periods.

In the present embodiment, a flashing phenomenon is prevented bycontrolling the output of the AC drive circuit 20. No new line orelement is used. Thus, the fineness of the subpixels SPX is not reduced.Further, the density of the circuit pattern is not increased, and thus,the yield ratio of manufacturing is not degraded.

In addition to the effects explained above, various excellent effectscan be obtained from the present embodiment.

Second Embodiment

In a second embodiment, a flashing phenomenon is difficult to becaptured by users with eyes since each storage period is short. Thedetails of the present embodiment are explained below.

FIG. 11 is a timing chart showing an example of an operation performedby a display device 1 according to the second embodiment. In a mannersimilar to that of FIG. 7, FIG. 11 assumes that each subpixel SPX is setto white display. FIG. 11 shows the potential of each signal in displayperiod 0, storage period 1, display period 1, storage period 2 anddisplay period 2 through time.

The waveform of each signal in each display period and each storageperiod is the same as that of FIG. 7. However, in FIG. 11, each storageperiod consists of a frame period TF1. Each display period consists offrame periods TF2. Each frame period TF1 is shorter than each frameperiod TF2 (TF1<TF2).

The frequency of a common signal VCOM and a auxiliary capacitance signalCS output by an AC drive circuit 20 in each storage period is a firstfrequency Fq1 (Hz). The frequency of the common signal VCOM and theauxiliary capacitance signal CS (FRP) output by the AC drive circuit 20in each display period is a second frequency Fq2 (Hz). Each frame periodTF1 is shorter than each frame period TF2. Thus, the first frequency Fq1is higher than the second frequency Fq2 (Fq1>Fq2). This frequency isbased on the number of waveforms from a specific value to the specificvalue through the maximum or minimum value as shown in FIG. 10.

In the example of FIG. 11, in a manner similar to that of FIG. 7, thedifference Vx in potential between a pixel electrode PE and a commonelectrode CE is large in storage period 2. In this manner, a flashingphenomenon may be produced. However, the first frequency Fq1 is higherthan the second frequency Fq2. Thus, the time in which the luminance isreduced is relatively short in storage period 2. For this reason, aflashing phenomenon is unnoticeable.

When the first frequency Fq1 is greater than or equal to 1.5 times thesecond frequency Fq2, it is possible to appropriately prevent thedegradation in display quality caused by a flashing phenomenon. When thefirst frequency Fq1 is greater than or equal to twice the secondfrequency Fq2, it is possible to obtain a higher effect of preventingthe degradation in display quality. To stably store potential in amemory 10, the first frequency Fq1 is preferably less than or equal to 5times the second frequency Fq2.

In addition, when the first frequency Fq1 is greater than or equal to 90Hz, users are difficult to notice a flashing phenomenon with eyes. Thus,it is possible to appropriately prevent the degradation in displayquality. The first frequency Fq1 is more preferably greater than orequal to 120 Hz since a flashing phenomenon is hardly visible in thisrange. The second frequency Fq2 may be determined as, for example,approximately 60 Hz. To stably store potential in the memory 10, thefirst frequency Fq1 is preferably less than or equal to 300 Hz.

In the above description, white display is explained. In a case of blackdisplay, similarly, a flashing phenomenon can be prevented.

In the present embodiment, a flashing phenomenon is prevented bycontrolling the output of the RC drive circuit 20 in a manner similar tothat of the first embodiment. No new line or element is used. In thisway, the fineness of subpixels SPX is not reduced.

Further, the density of circuit pattern is not increased, and thus, theyield ratio of manufacturing is not degraded.

In addition to the effects explained above, various excellent effectscan be obtained from the present embodiment.

Third Embodiment

In a third embodiment, a flashing phenomenon is prevented by allowing adisplay period to transition to a storage period when pixel potentialPIX satisfies a predetermined condition. The details of the presentembodiment are explained below.

FIG. 12 is a timing chart showing an example of an operation performedby a display device 1 according to the third embodiment. FIG. 12 assumesthat each subpixel SPX is set to white display in a manner similar tothat of FIG. 7. FIG. 12 shows the potential of each signal in displayperiod 0, storage period 1, display period 1, storage period 2 anddisplay period 2 through time.

In the present embodiment, when a display period transitions to astorage period, a control device 2 determines whether the transition ispossible. For this determination, potential Va, first potential V1,second potential V2 and third potential V3 are used. Potential Va ispixel potential PIX immediately before the start of each storage period.The first potential V1 is a low voltage VSS of a first drive signalxFRP. The second potential V2 is a high voltage VDD of the first drivesignal xFRP. The third potential V3 is the potential of a display signalSIG stored in a memory 10 in each storage period.

Specifically, the control device 2 determines that the transition to astorage period is possible when the following execution condition ismet: “potential Va is, out of the first potential V1 and the secondpotential V2, the potential having a smaller difference from the thirdpotential V3”. In this case, a display period transitions to the storageperiod, and the third potential V3 is stored in the memory 10. When theabove execution condition is not met, the control device 2 determinesthat the transition to the storage period is impossible. In this case,even if the write timing at which the display period is supposed totransition to the storage period has been reached, the display perioddoes not transition to the storage period. The display period isextended only by one or more predetermined frame periods TF (forexample, one frame period TF). The control device 2 determines againwhether the transition to the storage period is possible. When thetransition is possible, the display period transitions to the storageperiod.

The specific example is explained with reference to FIG. 12. The thirdpotential V3 to be stored in the memory 10 in storage period 1 is whitepotential VDD. In this case, out of the first potential V1 (VSS) and thesecond potential V2 (VDD), the potential having a smaller differencefrom the third potential V3 is the second potential V2, which has nodifference from the third potential V3. In display period 0 immediatelybefore write timing 1 at which storage period 1 should be executed,potential Va is the second potential V2. Thus, the above executioncondition is met, and the transition to the storage period is possible.At write timing 1, the operation of storage period 1 is performed.

In display period 1 immediately before write timing 2 at which storageperiod 2 should be executed, potential Va (in parentheses in the figure)is the first potential V1. In storage period 2, the third potential tobe stored in the memory 10 is VDD. In this case, the above condition isnot met. Thus, the transition to the storage period is impossible. Atwrite timing 2, the operation of storage period 2 is not performed.Display period 1 is extended only by one frame period TF.

Subsequently, in display period 1 immediately before write timing 2 a atwhich the operation of storage period 2 should be performed after theextension (in other words, in the extended period in the example of FIG.12), potential Va is VDD. In this case, the above execution condition ismet. Thus, the transition to the storage period is possible. In thisway, the operation of storage period 2 is performed at write timing 2 a.

In the example of FIG. 12, display period 1 is extended only by oneframe period TF when the execution condition is not met. However,display period 1 may be extended by more frame periods TF.

In the above description, it is assumed that all of the subpixels SPXincluded in a display area DA are set to white display. However, thesame control can be applied when all of the subpixels SPX are set toblack display, or when subpixels SPX for white display and subpixels SPXfor black display are mixed. When subpixels SPX for white display andsubpixels SPX for black display are mixed, subpixels SPX in whichvoltage Va is different from the third potential V3 are present at onewrite timing. In this case, for example, when the number of subpixelsSPX which satisfy the above execution condition is greater than or equalto a threshold out of all of the subpixels SPX, the display period maybe extended. Thus, the storage period may be put off. The flashingphenomenon in subpixels SPX for black display less affects the displayquality than that for white display. Thus, when the execution conditionis met in all of the subpixels SPX for white display, or when the numberof subpixels SPX which satisfy the above condition is greater than orequal to a threshold out of the subpixels SPX for white display, thedisplay period may be extended. Thus, the storage period may be put off.

In the present embodiment, the potential of the display signal SIG ineach storage period is close to (in the example of FIG. 12, is the sameas) the pixel potential PIX immediately before the storage period. Thus,even if the potential of a pixel electrode PE in a floating state in thestorage period changes in accordance with the potential of a commonelectrode CE, the large difference Vx in potential shown in FIG. 7 isnot produced. In this way, a flashing phenomenon can be prevented.

In addition to the effects explained above, various excellent effectscan be obtained from the present embodiment.

Fourth Embodiment

In the first to third embodiments, this specification discloses methodsfor solving the problems explained with reference to FIG. 7 to FIG. 9.Other problems which can be caused in a digital mode are explained usingthe timing chart of FIG. 13.

When a display period transitions to a storage period, a scanning signalGATEA is decreased to voltage VSS2, and switching elements Q1 and Q2 areoff. In this way, a select signal line 12 a is electrically disconnectedfrom a pixel electrode PE. Thus, the pixel electrode PE is set to afloating state. When the scanning signal GATEA is decreased, pixelpotential PIX could be also decreased by a predetermined potential ΔV bycapacitive coupling between the pixel electrode PE and a scanning lineG. In this case, the luminance of subpixels SPX changes in the storageperiod. Thus, the display quality is degraded.

To prevent the change in luminance, in the present embodiment, thepotential of a common signal VCOM and an auxiliary capacitance signal CSis decreased by a defined amount before switching elements Q1 and Q2electrically disconnect the select signal line 12 a from the pixelelectrode PE. The specific example of this operation is shown below.

FIG. 14 is a timing chart showing an example of an operation performedby a display device 1 according to the fourth embodiment. This timingchart is an example in which the present embodiment is applied to theoperation of the first embodiment. When a display period transitions toa storage period, an AC drive circuit 20 decreases the potential of thecommon signal VCOM and the auxiliary capacitance signal CS by a definedamount (ΔV in the example of FIG. 14) at a timing slightly earlier thanthe timing at which the scanning signal GATEA is decreased.

When the scanning signal GATEA is decreased, the pixel potential PIX isdecreased by potential ΔV by capacitive coupling between the scanningline G and the pixel electrode PE. After the scanning signal GATEA isdecreased, and the select signal line 12 a is electrically disconnectedfrom the pixel electrode PE, the AC drive circuit 20 increases thepotential of the common signal VCOM and the auxiliary capacitance signalCS by the defined amount ΔV. At this time, the pixel electrode PE is ina floating state. Thus, the pixel potential PIX is drawn to the increasein the potential of the common signal VCOM. Thus, the pixel potentialPIX is also increased by the defined amount ΔV.

By the above operation, the difference in potential between the pixelelectrode PE and the common electrode CE in each display period ismaintained in the storage period immediately after the display period.Thus, it is possible to prevent change in luminance caused by capacitivecoupling between the scanning line G and the pixel electrode PE.

In the example of FIG. 14, the defined amount is potential ΔV. However,the defined amount is not necessarily the same as potential ΔV. Evenwhen the defined amount is less than potential ΔV, the defined amountcould contribute to reduction in change in luminance in each storageperiod.

The AC drive circuit 20 may maintain the decrease in the common signalVCOM by the defined amount over the entire storage period.

In the example of FIG. 14, the present embodiment is applied to theoperation of the first embodiment. However, the present embodiment maybe applied to the operation of the second or third embodiment.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

For example, the structures disclosed in each embodiment may bearbitrarily combined.

In each embodiment, a display device having the functions of an analogmode and a digital mode is disclosed. However, the operation of thedisplay device of each embodiment may be applied to a display devicehaving only the function of a digital mode.

What is claimed is:
 1. A display device comprising: a pair of substrateshaving a display area in which a plurality of pixels are provided; aliquid crystal layer sealed between the pair of substrates; a pixelelectrode provided in each of the pixels; a common electrode which facesthe pixel electrode and produces a difference in potential for drivingthe liquid crystal layer between the common electrode and the pixelelectrode; a signal line to which a digital signal based on image datais supplied; a first drive circuit configured to supply the digitalsignal to the signal line; a second drive circuit configured to supplyan AC common signal to the common electrode; a memory provided in eachof the pixels in the display area; a first drive line to which a displaysignal of an image is supplied; a second drive line to which anon-display signal of an image is supplied; a storage control circuitconfigured to store the digital signal supplied to the signal line inthe memory in a storage period; and a select control circuit configuredto selectively supply, in a display period, to the pixel electrode, oneof the display signal and the non-display signal, the one correspondingto the digital signal stored in the memory, wherein a frequency of thecommon signal in the storage period is a first frequency, the frequencyof the common signal in the display period is a second frequency, andthe first frequency is higher than the second frequency.
 2. The displaydevice of claim 1, wherein the first frequency is greater than or equalto 1.5 times the second frequency.
 3. The display device of claim 1,wherein the first frequency is greater than or equal to twice the secondfrequency.
 4. The display device of claim 2, wherein the first frequencyis less than or equal to five times the second frequency.
 5. The displaydevice of claim 1, wherein the first frequency is greater than or equalto 90 Hz.
 6. The display device of claim 1, wherein the first frequencyis greater than or equal to 120 Hz.
 7. The display device of claim 5,wherein the first frequency is less than or equal to 300 Hz.
 8. Thedisplay device of claim 1, wherein the select control circuit comprises:a select signal line which is connected to one of the first drive lineand the second drive line, the one corresponding to the digital signalstored in the memory; and a switching element configured to electricallyconnect the select signal line and the pixel electrode or toelectrically disconnect the select signal line from the pixel electrode,and the switching element electrically disconnects the select signalline from the pixel electrode in the storage period.
 9. The displaydevice of claim 8, wherein when the display period transitions to thestorage period, the second drive circuit decreases a potential of thecommon signal by a defined amount before the switching elementelectrically disconnects the select signal line from the pixelelectrode.
 10. The display device of claim 9, wherein after the seconddrive circuit decreases the potential of the common signal by thedefined amount, and the switching element electrically disconnects theselect signal line from the pixel electrode, the second drive circuitincreases the potential of the common signal by the defined amount. 11.The display device of claim 1, wherein a polarity of a potential betweenthe pixel electrode and the common electrode is periodically inverted inthe display period.
 12. The display device of claim 1, wherein thesignal line extends, passing between the adjacent pixels, at least oneof the substrates comprises a light-shielding layer which blocks light,and the light-shielding layer does not overlap the signal line betweenthe adjacent pixels.
 13. The display device of claim 1, wherein thesubstrates include a first substrate and a second substrate, the pixelelectrode is provided in the first substrate, the first substratecomprises a reflective layer which reflects, toward the secondsubstrate, light which has reached the first substrate from the secondsubstrate, and an image is displayed by the light reflected by thereflective layer.
 14. The display device of claim 1, further comprisinga digital mode and an analog mode, wherein the first drive line is thesame line as the signal line, operations of the storage period and thedisplay period are performed in the digital mode, and a signal based ona gradation of an image is supplied to the first drive line in theanalog mode.
 15. The display device of claim 1, further comprising acapacitance line which forms a capacitance for driving the liquidcrystal layer between the capacitance line and the pixel electrode, andthe second drive line is the same line as the capacitance line.